What is a gag gift for someone who hit their first home run?

An autographed baseball from the pitcher who served it up!!

Those green shopping bags? Get four of them. Give him a four-bagger.

Some electrical components, to complete a circuit.

Were the bases empty? Give him a souvenir (non-functional) bullet, so he can remember his solo shot.

Bases loaded? Treat him to a Grand Slam breakfast at Denny's.

Two men on? Splurge for a trifecta ticket at Aqueduct, so he can watch three runs of the races.

And, of course, a round-trip ticket on any affordable airline.

Go to the store and get some jelly beans. Take an old prescription bottle of medicine and put the beans in it and give it to him and say he needs to 'roid' up more. It puts a light spin on baseballs black eye.

A car "jack." :)